An electron emitter assembly and a method for adjusting a size of electron beams are provided. The electron emitter assembly includes a laser configured to emit a first light beam. The electron emitter assembly further includes a lens assembly configured to receive the first light beam. The lens assembly is configured to adjust a size of the first light beam between a first predetermined size and a second predetermined size larger than the first predetermined size. The lens assembly emits the first light beam toward a photo-cathode. The photo-cathode is configured to emit a first electron beam having a third predetermined size when the first light beam having the first predetermined size contacts the photo-cathode. The photo-cathode is further configured to emit a second electron beam having a fourth predetermined size when the first light beam having the second predetermined size contacts the photo-cathode. The electron emitter assembly further includes an anode configured to receive the first and second electrons beams from the photo-cathode.